Twitter and Reddit user @MetricMaps has developed a GIF that shows the steady rise of America's state and federal prison population from 1978 to 2012. The map shows that the South — and Nevada — were leaders in increasing incarceration, but that most of the rest of the country has followed.
Incarceration rates are mostly due to government policies, not to crime rates — that's true at the national level, and it's true for differences between states as well. And because most of the US prison population is housed in state prisons, state laws, in particular, are the biggest factor in the rise of mass incarceration — and differences between similar states can be explained by differences in their laws.
As legal historian Sara Mayeux wrote in 2010:
Incarceration rates are largely driven by policy choices, not crime rates or demographics. Wisconsin and Minnesota have similar demographics and crime rates but Wisconsin has a much larger prison population. A similar disparity exists between North Dakota and South Dakota.
However, demographics are certainly an important part of the story, given that African American and Hispanic men are incarcerated at such a higher rate than other groups. The fact that the trend toward mass incarceration started in the Deep South is evidence for Michelle Alexander's thesis that mass incarceration is "the new Jim Crow": a set of policies that work together to control the black population.
Even states with low overall incarceration rates can still be guilty of racial disparities in enforcement. In fact, the states with the biggest racial disparities in marijuana arrests between blacks and whites are all in the Northeast and Upper Midwest, not in the South.
CORRECTION: This article originally displayed a map that showed the incarceration rates of female prisoners. We apologize for the error.